Rhesus Incompatibility

The name rhesus refers to various types of rhesus antigens on the surface of red blood cells. The antigens on the red blood cells vary between individuals. The rhesus antigens are separate to the ABO blood group system.

Within the rhesus group, many different types of antigens can be present or absent, depending on the person’s blood type. The most relevant antigen within the rhesus blood group system is the rhesus-D antigen. When we refer to someone’s rhesus status in relation to pregnancy (e.g. “she is rhesus-negative”), we are usually referring to whether they have the rhesus-D antigen present on their red blood cell surface.


Rhesus Incompatibility in Pregnancy

Women that are rhesus-D positive do not need any additional treatment during pregnancy.

When a woman that is rhesus-D negative becomes pregnant, we have to consider the possibility that her child will be rhesus positive. It is likely at some point in the pregnancy (i.e. childbirth) that the blood from the baby will find a way into the mother’s bloodstream. When this happens, the baby’s red blood cells display the rhesus-D antigen. The mother’s immune system will recognise this rhesus-D antigen as foreign, and produce antibodies to the rhesus-D antigen. The mother has then become sensitised to rhesus-D antigens.

Usually, this sensitisation process does not cause problems during the first pregnancy. During subsequent pregnancies, the mother’s anti-rhesus-D antibodies can cross the placenta into the fetus. If that fetus is rhesus-D positive, these antibodies attach themselves to the red blood cells of the fetus and causes the immune system of the fetus to attack them, causing the destruction of the red blood cells (haemolysis). The red blood cell destruction caused by antibodies from the mother is called haemolytic disease of the newborn.



Prevention of sensitisation is the mainstay of management. This involves giving intramuscular anti-D injections to rhesus-D negative women. There is no way to reverse the sensitisation process once it has occurred, which is why prophylaxis is so essential.

The anti-D medication works by attaching itself to the rhesus-D antigens on the fetal red blood cells in the mothers circulation, causing them to be destroyed. This prevents the mother’s immune system recognising the antigen and creating it’s own antibodies to the antigen. It acts as a prevention for the mother becoming sensitised to the rhesus-D antigen.

Anti-D injections are given routinely on two occasions:

  • 28 weeks gestation
  • Birth (if the baby’s blood group is found to be rhesus-positive)


Anti-D injections should also be given at any time where sensitisation may occur, such as:

  • Antepartum haemorrhage
  • Amniocentesis procedures
  • Abdominal trauma


Anti-D is given within 72 hours of a sensitisation event. After 20 weeks gestation, the Kleinhauer test is performed to see how much fetal blood has passed into the mother’s blood, to determine whether further doses of anti-D are required.


Kleihauer Test

The Kleihauer test checks how much fetal blood has passed into the mother’s blood during a sensitisation event. This test is used after any sensitising event past 20 weeks gestation, to assess whether further doses of anti-D is required.

The Kleihauer test involves adding acid to a sample of the mother’s blood. Fetal haemoglobin is naturally more resistant to acid, so that they are protected against the acidosis that occurs around childbirth. Therefore, fetal haemoglobin persists in response to the added acid, while the mothers haemoglobin is destroyed. The number of cells still containing haemoglobin (the remaining fetal cells) can then be calculated.


Last updated September 2020